Species Streblospio eridani n. sp. (Figs 2-6) Type material. Holotype: Sacca di Goro (2nd October 2017), complete specimen (MNHF I.AL.19.0002-1), collected at Station C 3 (44°47.717’N, 12°20.620’E) by C. Munari in muddy sediment between 0.5 and 1.7 m depth (Table 1). Paratypes: Sacca di Goro (October 2017), 9specimens (MNHF I.AL.19.0002-2/10),collected at P1 (44°49.758’N, 12°18.105’E); 5 specimens (MNHF I.AL.19.0002-11/15) collected at station P2 (44°48.676’N, 12°20.748’E); 3 specimens (MNHF I.AL.19.0002-16/18) collected at station P5 (44°47.793’N, 12°22.177’E); 10 specimens (MNHF I.AL.19.0002-19/28) collected at station C1 (44°49.061’N, 12°19.395’E); 4 specimens (MNHF I.AL.19.0002-29/32) collected at station C5 (44°47.435’N, 12°22.328’E); 2 specimens (MNHF I.AL.19.0002-33-34) collected at station C2 (44°47.783’N, 12°19.422’E). Valle di Gorino (June 2009), 4 specimens, (MNHF I.AL.19.0002-35/38) collected at station G2 (44°47.481’N, 12°21.262’E). Valli di Comacchio, June 2013, one specimen (MNHF I.AL.19.0002-39) collected at station C2 (44°37.944’N, 12°7.375’E); May 2014, 2 specimens (MNHF I.AL.19.0002-40/41) collected at station B1 (44°36.912’N, 12°12.731’E). Lido di Dante (September 2017), 3 specimens (MNHF I.AL.19.0002- 42/44) collected at station ID (44°23.168’N, 12°19.294’E). Description. Holotype complete specimen (Fig. 2a), 5.10 mm long, 0.39 mm wide (including parapodia) at chaetiger 6, with 40 chaetigers; female, with oocytes, and brooding structures between chaetigers 15 and 26. Paratypes complete (Fig. 2b), in number of 26, ranging from 36 to 50 chaetigers, 4.74–14.93 mm long, and 0.22–0.45 mm wide (including parapodia), complete although some paratypes lost palps or branchiae during handling and examination. Paratypes incomplete, in number of 17, 0.22–0.44 mm wide, with 8–32 chaetigers. Prostomium elongate and anteriorly rounded (Fig. 2c), partially enveloped by peristomial extensions; presence of prostomial peaks (sensory knobs similar to papillae) along the fronto-lateral edge of the prostomium, on the dorsal and ventral surface (Fig. 2c), and easily visible even with a light microscope (Fig. 2d). Two pairs of brownish, small eyes in a trapezoidal arrangement (Fig. 2d); a few specimens with 5 eyes. Eyes no longer visible in some specimens preserved in alcohol. One pair of palps situated on midlateral side of prostomium (Fig. 3a), ciliated on frontal surfaces (Fig. 3b), extending to chaetigers 6 in the holotype, and to chaetigers 5–10 in paratypes. Palps have three groups of cilia (i.e., lateral cilia, frontal cilia and latero-frontal cirri; Fig. 3c, d). Peristomium with short dorsolateral wings (Fig. 2b), surrounding base of palps (Fig. 2c), and fused with the first chaetiger. One pair of thick, ciliated branchiae inserted just posterior to palps (on chaetiger 1), extending back to chaetigers 5 in the holotype, and to chaetigers 5–11 in paratypes, with distal digitiform appendage (Fig. 4a, d); slightly thicker than palps. Branchiae with central axis and two laterally flattened surfaces (Fig. 4a, b, c); frontal surface densely ciliated on inner part (oriented towards midline of body) and sparsely ciliated on its outer part; abfrontal surface sparsely ciliated with two main tracts of cilia along long axis of branchia (Fig. 4b). A few specimens having branchiae with transverse bands of dark pigment disappearing after fixation over time. Length of palps and branchiae depends on state of contraction or expansion. A small conical dorsal papilla (like a small occipital antenna) on chaetiger 1 between branchiae (Fig. 2c), difficult to see without removing them. Parapodia on chaetiger 1 with neuropodial postchaetal lamellae only, neuropodial lamellae protruding, small flap-like, ear-shaped (Fig. 2c, 4a, 6a), and slightly inclined; notopodial lamellae lacking, and only few capillaries present. Notopodium of chaetiger 2 with broad postchaetal lamellae continuous with dorsal hood extending across dorsum connecting left and right notopodia (Figs 2c, 4a, 6a), and forming a high dorsal entire membranous collar (Figs 4a, 6a); anterior margin of hood widens upward and forms a pouch open anteriorly (Figs 4a, 6a); notopodial postchaetal lamellae of chaetiger 2 rounded on ventral edge. Neuropodium of chaetiger 2 with broad flap-like postchaetal lamellae, rectangular shaped (Figs 2c, 6a). Neuropodial lamellae rounded from chaetiger 3. Notopodial and neuropodial postchaetal lamellae small and flap-like on anterior chaetigers, and absent on posterior chaetigers: notopodial postchaetal lamellae progressively reducing from chaetiger 8 until they disappear beyond chaetigers 11- 18; neuropodial lamellae progressively reducing from chaetiger 7 until disappearing between chaetigers 10 and 12. Neuropodial and notopodial prechaetal lamellae absent. All notopodial chaetae capillary; neuropodial chaetae include capillaries, and 1-2 sabre chaetae and multidentate hooded hooks; capillaries on anterior chaetigers are unilimbate and slightly hirsute (Fig. 5a, b), posteriorly slender, alimbate and less hirsute. Chaetiger 1 with 1–4 capillary notochaetae (Figs 2c, 4a, 6a) (2 capillaries in holotype), and 3-6 neuropodial capillary chaetae (4 capillaries in holotype) (Figs 2c, 4a, 6a). Notopodium of chaetiger 2 with 5-11 capillary chaetae (8 capillaries in holotype); neuropodium of chaetiger 2 with 4-9 capillary chaetae (7 capillaries in holotype). Chaetigers 3 to 6 with 6-10 notopodial (7-9 capillaries in holotype) and 7-11 neuropodial (8-9 capillaries in holotype) capillary chaetae. Notopodium of chaetiger 7 with only 6-9 capillary chaetae (8 capillaries in holotype). Neuropodial hooded hooks and sabre chaetae first appearing on chaetiger 7, and continuing almost to end of body; each hook with 4-5 pairs of small teeth in addition to main unpaired tooth (Fig. 6b); without secondary hoods. Neuropodium of chaetiger 7 with 2-4 hooded hooks (3 hooks in the holotype) accompanied by 2-4 slender capillary chaetae in a single row (2 capillaries in the holotype), and 1-2 sabre chaetae (1 sabre chaeta in the holotype). Sabre chaeta stout, curved, alimbated, and slightly granulated. On the middle body region, chaetigers 20-24, notopodia with only 4-6 capillary chaetae; neuropodia with 3-5 hooded hooks, 2-3 companion capillary chaetae and 1-2 sabre chaetae (Fig. 6c). On posterior region of the body, notopodia with 2-5 thin long alimbate capillary chaetae (3 in holotype); neuropodia with 6-10 hooded hooks (8 hooks in holotype), 1-3 alimbate capillaries and one sabre chaeta (Fig. 6d). Last three chaetigers with notopodia having only 2 long and slender capillaries, and neuropodia without companion capillaries. The second-last chaetiger with neuropodia having 4-6 hooded hooks (4 in holotype) and up to one sabre chaeta (Fig. 6d); the last chaetiger with neuropodia having 2-4 hooded hooks (2 in holotype) and without sabre chaetae. Pygidium with one pair of ventral lappets, without anal cirri (Fig. 6d). Finger-like extensions of lateral body wall in females (200 specimens examined, 23 out of these deposited as paratypes (all female) present from chaetigers 13–18 to chaetigers 27-28 (15-26 in holotype); extensions oriented dorsally and posteriorly, and resembling branchiae (Figs 2b, 7a); the base of these brood structures similar to pouches arising between noto- and neuropodium. Oogenesis intraovarian, it begins in chaetigers anterior (ahead) to those of the first branchiate brooding structures. Oocytes first occur laterally (i.e., in a lateral row), within paired ovaries attached to genital blood vessels, from chaetigers 7-8. In the subsequent chaetigers oocytes are present at the base and within the branchiate brooding structures (Fig. 7b, c); oocytes of different sizes, slightly oval-shape, 25.82 - 135.56 µm (n = 230) in diameter. The brooding structures contain about two to three eggs. Posterior to branchiate region on females are three or four dorsal appendages in rows between left and right notopodia (Fig. 7d); these extensions present on posterior chaetigers to near end of body. Methyl green staining pattern. Specimens of the new species herein described exhibit methyl green staining reaction. Prostomium and the anterior half (first 8-9 chaetigers) of the body are stained intensely, with darker dorsolateral and ventral bands of stain; the staining has a discoloration on the dorsal pouch (Fig. 7e). Type locality. Sacca di Goro, Po Delta lagoon, Emilia Romagna, Italy, Adriatic Sea. Habitat and ecology. Adults of Streblospio eridani n. sp. were collected between 0.5 and 3.5 m depth in sand, silty clay, clayey silt and clay sediments (Table 1). In the eutrophic Sacca di Goro, this species became numerically dominant in the community in October 2017, and it occurred in silty clay sediments to a depth of 1.7 m, at sampling stations characterized by wide salinity range (4.5-33.7 PSU). In that period, its density ranged from 986.7 ind. m-2 (Station P2) to 21,213.3 ind. m-2 (Station C5), and it accounted for 52-97.8 % of the total abundance of the benthic macroinvertebrates (Table 1). Streblospio eridani n. sp. was associated with Chironomus salinarius Kieffer, 1915, indicator of organic enrichment and stagnant waters; other abundant species were tolerant or opportunistic, such as spionid polychaetes, Capitella Blainville, 1828 species complex, Hydroides dyanthus (Verrill, 1873) and the bivalve Arcuatula senhousia (Benson, 1842). Re-examination of old material from the Valli di Comacchio (dating back to 2013, in clayey sediments, 1.5 m depth) and the Valle di Gorino (dating back to 2009), revealed that some specimens (6 and 4, respectively), originally identified as Streblospio sp., in fact belong to S. eridani n. sp., and that some of them (i.e., two specimens from the Valle di Gorino, and one from the Valli di Comacchio) had oocytes. In the Valle di Gorino and Valli di Comacchio the new species occurred in low density (Table 1). The community of the Valli di Comacchio in which the new species occurred was extremely similar to that of the Sacca di Goro, and characterized by opportunistic (e.g., spionid polychaetes, oligochaetes and chironomids) and tolerant species (i.e., the bivalve Cerastoderma galucum (Bruguière, 1789)). The community of the Valle di Gorino was characterized by stress tolerant species such as the amphipod Corophium orientale Schellenberg, 1928, the gastropod Ecrobia ventrosa (Montagu, 1803), and the polychaete Hediste diversicolor (O.F. Müller, 1776). In the marine site, Lido di Dante, S. eridani n. sp. occurred in sandy sediments in low density, representing only from 0.1% (station ID August and September 2017) to 0.5% (station ID September 2016) of the total abundance of the benthic macroinvertebrate community. Its density ranged from 12.3 ind. m-2 (stations: OD August 2016, ID August 2017, ID September 2017) to 86 ind m-2 (station ID September 2016). These sampling stations were characterized during the study periods by salinities of 33.2-37.3 PSU. Also at the Lido di Dante, Streblospio eridani n. sp. occurred in association with other opportunistic spionids belonging to the genera Minuspio Foster, 1971, Prionospio Malmgren, 1867 and Polydora Bosc, 1802. ind. m-2 + SE = average individuals m−2 ± standard error Remarks. Recently, Delgado-Blas et al. (2018) suggested raising the status of Streblospio benedicti japonica Imajima, 1990 to full species level as S. japonica Imajima, 1990 on the basis of several morphological characters (i.e., number of segments, segment on which sabre chaetae occur, number of accessory teeth on neuropodial hooks, and presence of ventral lappets on pygidium). Thus, with the addition of S. eridani n. sp., the number of described species in Streblospio would increase to seven. Among the Streblospio species previously described, four seem to be native to European waters: the new species herein described, S. shrubsolii (Buchanan, 1890), S. eunateae Martínez & Adarraga, 2019 and S. padventralis Delgado-Blas, Díaz-Díaz & Viéitez, 2018. However, the last two species have not yet been recorded from the Mediterranean Sea. Similarly, despite S. benedicti Webster, 1879 having been introduced to European coasts (Carlton 1979; Fonseca-Genevois & Cazaux 1987), it has not been reported from the Mediterranean Sea. Interestingly, S. japonica has never been found outside its native biogeographic region. The species placed within the genus Streblospio display slight morphological differences although there are considerable differences in their reproductive biology (Mahon et al. 2009). For example, males of S. gynobranchiata Rice & Levin, 1998 are morphologically similar to S. benedicti, but females differ in reproductive structures, that is for the lateral body wall extensions resembling branchiae in S. gynobranchiata and for the specialized dorsal brooding pouches in S. benedicti (Rice & Levin 1998). Schulze et al. (2000) on the basis of genetic and morphological analyses even hypothesized a historical hybridization between S. benedicti and S. gynobranchiata followed by repeated backcrossing of the hybrids to S. benedicti, resulting in the transfer of S. gynobranchiata haplotype(s) to an S. benedicti morphology. Moreover, a wide variability in the morphological characters was found in the species of the genus Streblospio, and with considerable overlap in ranges. For example, a certain overlap is reported in the first and last gametogenic chaetiger in females and for this reason Schulze et al. (2000) suggested using caution when using the reproductive morphology alone to identify the species. Likewise, some morphological characteristics of S. eridani n. sp. displayed an intraspecific variability, such as the beginning of gametogenic chaetigers, the number of sabre and capillary chaetae, the number of hooded hooks. In specimens of S. eridani n. sp. such a variability of morphological characters was also found between body regions. Further difficulties in the morphological identification derive from the differences in the descriptions of specimens and/or populations from different geographical areas of S. benedicti (Webster 1879; Hartman 1936; Foster 1971), S. gynobranchiata (Rice 1984) and S. shrubsolii (Buchanan 1890; Horst 1909; Fauvel 1927). For example, Horst (1909) described S. dekhuyzeni Horst, 1909 with two types of capillary notochaetae and more hooded hooks per ramus than S. shrubsolii. However, these differences were minor enough and S. dekhuyzeni was considered synonymous with S. shrubsolii by Fonseca-Genevois & Cazaux 1987. Hartman (1936) described Streblospio lutincola Hartman, 1936 with much smaller ventral peristomial fold, and much larger prostomium than S. benedicti, and S. lutincola was later synonymized with S. benedicti (Hartman 1944). Streblospio eridani n. sp. is similar to the original description (Rice & Levin 1998) of S. gynobranchiata in having branchiae with distal digitiform appendage, branchiate brooding structures, dorsal collar entire on chaetiger 2, presence of a dorsal papilla between branchiae, and beginning from chaetiger 7 of both sabre chaetae and hooded hooks. However, S. eridani n. sp. can be distinguished by the shape of posterior dorsal appendages on female, be- ginning (i.e., first branchiate structures at chaetiger 13 in the new species and chaetiger 20 in S. gynobranchiata) and number of paired branchiate structures (i.e., for about 10-12 chaetigers in S. eridani n. sp. and for about 15 chaetigers in S. gynobranchiata), as well as in that the new species has: 4-5 pairs of teeth in the neuropodial hooks rather than 3-4 rows of teeth, absence of postchaetal lamellae on posterior chaetigers rather than digitiform ones, and pygidium with lappets rather than simple, as specified in the original description of S. gynobranchiata by Rice & Levin (1998) and in the subsequent description by Çinar et al. (2005a). Besides the presence of branchiate brooding structures (rather than transverse, mid-segmental ridges), the new species is easily distinguished from the other congeneric species from the Mediterranean, that is S. shrubsolii, by several characters: shape of pygidium (with lappets rather than simple, that is without such lobes, as in the latter species), beginning of neuropodial hooks from chaetiger 7 rather than from chaetigers 8-10, number of small teeth in the neuropodial hooks, the first gametogenic chaetiger (19 in the latter species, rather than in chaetigers 7-8), presence of dorsal papilla and sabre chaetae, which are both absent in S. shrubsolii even though the latter species presents a low raised elevation on the first chaetiger. Finally, S. eridani n. sp. differs from S. shrubsolii also in the distribution of prostomial peaks: it has such sensory knobs (like papillae) restricted to the fronto-lateral edge of the prostomium, rather than being widely scattered on all surfaces of the prostomium as observed in S. shrubsolii by Dauer et al. (2003). Streblospio eridani n. sp. differs from S. padventralis in that in the former species sabre chaetae begin from chaetiger 7 rather than from chaetiger 3, and dorsal (occipital) papilla is present, rather being absent. The shape of prostomium is a further character useful to differentiate this new species from S. padventralis, in that the latter species has a subdistal pad surrounding the ventral region. Streblospio eridani n. sp. is easily distinguished from the description of S. eunateae (Martínez & Adarraga 2019) by its smaller size (4.7 - 14.9 mm, compared to 10 - 25 mm in S. eunateae), and the staining pattern with methyl green, in that the latter species shows uniform staining thoroughout the body, rather than a defined pattern (as in the new species herein described). Streblospio eridani n. sp. differs from S. eunateae also in that the latter has: neuropodial hooded hooks from chaetigers 9-10, rather than from chaetiger 7; oogenesis beginning from chaetiger 20, rather than 7-8; presence of incubator chambers without extensions resembling branchiae, rather than with such structures (as in this new species). About the term nuchal antenna (or papilla) used by Rice and Levin (1998), in agreement with Delgado-Blas et al. (2018) and Radashevsky (2012) we believe that it is unsuitable as this appendage is located dorsally on the first chaetiger between the two branchiae. Following these authors (Radashevsky 2012; Delgado-Blas et al. 2018) we referred to this structure to as dorsal papilla or occipital antenna. On the basis of the revision of the genus Streblospio provided by Rice & Levin (1998) and the recent descriptions of two new species of Streblospio by Delgado-Blas et al. (2018), and Martínez & Adarraga (2019), we summarize in Table 2 the main morphological, reproductive and ecol